Several key issues and insights in full-scale proppant-support fracturing for deep coal rock gas formations

Our country has abundant coal rock gas resources. Hydraulic fracturing is a key technology for effectively developing coal rock gas reservoirs. Due to the differences in the mechanical properties, microstructure, gas occurrence states, and productivity-controlling factors of deep coal rock compared with shallow and medium-depth coal reservoirs, as well as the significant property variation in reservoirs between blocks, the adaptability of current fracturing technologies still faces challenges. Innovation in reservoir stimulation technologies is essential for the efficient development of coal rock gas. The difficulties in reservoir stimulation brought by the geological characteristics of coal rock gas reservoirs are discussed first. To deal with the geological features and the challenges of fracturing technology, an efficient development concept of matrix pore-cleat/fracture simultaneous stimulation, summarized as “point desorption, line dredging, fracture geometry improvement, and propped bulk fracture network”, is proposed, and the following key issues are figured out based on its connotation: ① fracturing-induced matrix pore structure and pore surface property modification, enhanced gas desorption, imbibition displacement, and adsorbed-phase and free-phase methane collaborative and efficient gas supply; ② activate cleats/fractures, shorten gas diffusion distances, and facilitate matrix reserves releasing; ③ promote uniform fracture propagation, enhance fracture complexity, and precisely control fracture morphology; ④ full-scale proppant support for coal rock reservoirs that “precisely matches proppant particle size with multi-level fracture widths, supports bedding-plane fractures, and provides three-dimensional support for cleat and main fractures”. Results indicate that it is necessary to further study the relationship between fracture parameters and production dynamics based on the gas storage and production characteristics of deep coal rocks, in order to identify fracture parameters that can realize the adsorbed and free gas “continuous-cooperative” supply, and to provide support for fracture property control and treating design optimization. Conduct in-depth research on hydraulic fracture network propagation rules in coal rock reservoirs and fracture propagation numerical simulation technologies, and combine the net pressure log-log diagram during fracturing to effectively control the fracture network propagation behavior in coal rock gas reservoirs. Use high-viscosity and leakage-weakening fluid first to create the main fractures, then use low-viscosity fluid to create complex fractures, achieving “controlled near-wellbore fracture complexity and sufficiently extended fractures” to form a “long fracture network”. To deal with the requirement of high conductivity and larger volume for fracture networks in deep coal rock formations, maximizing fracture volume and optimizing flow capacity with limited proppant and fluid is an effective way to reduce costs and increase efficiency. Propose the full-scale proppant-support fracturing technology for deep coal rock reservoirs to achieve long-term connectivity of “main fractures + bedding planes + cleats” and increase the effective support volume of fractures. Optimize different proppants and fiber combinations through long-term fracture conductivity tests for multi-size fractures with different proppant placement patterns. Improve existing fracturing fluid systems, explore water-reducing, high-sand-ratio, low-cost fracturing fluids, and clean desorption-promoting agents.